The lymphoselective toxicity of 2-chloro-2′-deoxyadenosine (CldAdo, cladribine) and its potential as a chemotherapeutic agent against lymphoid neoplasms were reported by Carson et al.1 This potent, deaminase-resistent analogue of 2′-deoxyadenosine (dAdo) is currently the drug of choice for hairy-cell leukemia.2,3 It also has significant activity against chronic lymphocytic leukemia,4,5 indolent non-Hodgkin's lymphoma,6 and Waldenström's macroglobulinemia.7 Investigations with cladribine treatment of multiple sclerosis,8 systemic lupus erythematosis-associated glomerulonephritis,9 and other rheumatoid and immune disorders are in progress. Cladribine is a nucleoside prodrug, which is phosphorylated by deoxycytidine kinase to CldAMP, and then sequentially to CldADP and the active CldATP.1a,10a Cladribine also is a good substrate for mitochondrial 2′-deoxyguanosine (dGuo) kinase,10 and induction of programmed cell death by direct effects on mitochondria has been implicated in its potent activity against indolent lymphoid malignancies (via apoptosis) as well as in proliferating cells.11,12 
Various methodologies have been published for the production of Cladribine. Venner reported Fischer-Helferich syntheses of naturally occurring 2′-deoxynucleosides in 1960,13 and employed 2-chloro-2′-deoxyadenosine as an intermediate for 2′-deoxy(guanosine and inosine). Ikehara and Tada also synthesized dAdo with CldAdo as an intermediate [obtained by desulfurization of 8,2′-anhydro-9-(β-D-arabinofuranosyl)-2-chloro-8-thioadenine].14 
Syntheses of CldAdo as a target compound have exploited the greater reactivity of leaving groups at C6 relative to those at C2 of the purine ring in SNAr displacement reactions. Robins and Robins15 employed fusion coupling of 2,6-dichloropurine with 1,3,5-tri-O-acetyl-2-deoxy-α-D-ribofuranose. The 9-(3,5-di-O-acetyl-2-deoxy-α-D-erythro-pentofuranosyl)-2,6-dichloropurine anomer was obtained by fractional crystallization. Selective ammonolysis at C6 and accompanying deprotection gave 6-amino-2-chloro-9-(2-deoxy-α-D-erythro-pentofuranosyl)purine. The pharmacologically active β-anomer (cladribine) was prepared by an analogous coupling, chromatographic separation of anomers, and ammonolysis.16 
Stereoselective glycosylation of sodium salts of halopurines and analogues with 2-deoxy-3,5-di-O-p-toluoyl-α-D-erythro-pentofuranosyl chloride gave β-nucleoside anomers via predominant Walden inversion,17,18 and ammonolysis/deprotection gave CldAdo.19 Although the sodium salt glycosylation usually gave good anomeric stereoselectivity, minor quantities of α anomers and >10% of N7 regioisomers were usually formed.20,21 This requires separations and results in diminished yields of the desired N9 product. Carson et al.1 had reported an enzymatic transfer of the 2-deoxy sugar from thymidine to 2-chloroadenine (ClAde). Holy and coworkers noted that cells of a strain of Escherichia coli performed glycosyl transfer from 2′-deoxyuridine to 2-chloro-6-[(dimethylaminomethylene)amino]purine to give a derivative of CldAdo.22 Very recently Barai, Mikhailopulo, and coworkers23 described an E. coli-mediated glycosyl transfer synthesis of 2,6-diamino-9-(3-deoxy-β-D-erythro-pentofuranosyl)purine,24 and its enzymatic deamination to 3′-deoxyguanosine.24 They reported glycosyl transfer from 2′-deoxyguanosine to ClAde, and claimed a yield of 81% of CldAdo (based on ClAde).23 However, a 3:1 ratio of dGuo/ClAde was employed, so the yield of CldAdo was <27% based on dGuo.
Sampath et al. have recently shown (U.S. Pat. No. 6,596,858 B2) a method for making 2-chloro-2′-deoxyadenosine compounds, using 2-amino-2′-deoxyadenosine as a starting compound, but beginning with an initial diazotization/chloro-dediazoniation reaction on the unprotected nucleoside to replace the 2-amino group with a 2-chloro group. This method, however, creates various impurities, which requires extensive purification procedures, and results in an overall yield of only 27%.
Accordingly, there is a significant need to produce CldAdo using methods that result in a higher yield, are more cost effective, and result in a more purified form.